Foam compositions with high amounts of alcohol are known in the art. Alcohol-based compositions are useful because of the anti-microbial properties of alcohol and the ability for alcohol to dissolve certain active agents.
Foams and, in particular, single-phase foams are complicated systems which do not form under all circumstances. Slight shifts in foam composition, such as by the addition of active ingredients or the removal of any of the essential ingredients, may destabilize the foam.
The prior art teaches hydro-alcoholic foam compositions require significant amounts of short-chain alcohols (namely, ethanol, propanol, isopropanol, butanol, iso-butanol, t-butanol and pentanol), water, fatty alcohols, polymer and surfactant to form a foam. These compositions require various surfactants, such as, non-ionic surfactants, anionic, cationic, zwitterionic, amphoteric and ampholytic surfactants, as essential components.
Surfactants are known as essential ingredients in foam compositions because of their amphiphilic properties and because they are considered essential in forming a foam. However, many surfactants are known to be irritating when left on the skin, as they can extract lipids from the skin, thereby damaging skin barrier and exposing the skin to contact with pro-inflammatory factors. (See, Dermatitis, Vol. 33(4) 217-225, 11 Apr. 2006, John Wiley & Sons).
Lower alcohols are defatting agents. They are known to extract skin fats, thereby disrupting skin barrier function and causing irritation. They are known to cause skin to become dry and cracked (See, for example, Industrial Guide to Chemical and Drug Safety, by T. S. S. Dikshith, Prakash V. Diwan, John Wiley & Sons, Inc., 2003, p. 228-9).
Thus the combination of a short chain alcohol and a surfactant can have a doubly undesirable irritating and defatting effect, as well as the drawback of enhanced delivery of drugs through the skin, which results in increased systemic exposure (which is undesirable for topical treatment of the skin).
Hydro-alcoholic foams, as described in the prior art are inherently thermally unstable, and they will collapse upon exposure to the skin and body (at temperatures around 37° C.). They are therefore commonly termed “quick breaking” foams. Typically, when a quick breaking foam is applied to fingers (as is usually done in order to apply a drug to a target area), it melts and rapidly (on exposure to body temperature of about 37° C.) and collapses leaving behind a small pool of liquid. The thermal instability of the foam makes it difficult to apply to a large target area by first administering the foam to the hands and then spreading the foam onto the affected area.